On March 29, 1974, NASA’s Mariner 10 spacecraft has performed the first Mercury flyby in the history of space exploration.

March 29 story of what happened this day in Science, Technology, Astronomy, and Space Exploration history.

Mariner 10

Mariner 10 was a robotic space probe launched by NASA on November 3, 1973, to study Mercury and Venus. It was the seventh successful launch in the Mariner program and the first spacecraft to visit Mercury. The mission was managed by NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. It was also the first spacecraft to perform flybys of multiple planets (Venus and Mercury – Mariner 10 used the gravity assist of Venus to modify its speed and trajectory, enabling it to reach Mercury).

In 1973, a unique opportunity arose to send a spacecraft on a single mission to explore both Venus and Mercury. By utilizing a technique known as a gravity assist, which had been theorized for decades but never implemented, a spacecraft could use the gravitational pull of one planet to propel itself toward the next. This approach eliminated the need for additional fuel and a larger rocket that would have been required to launch a heavier spacecraft, as well as reduced the time needed to reach the final destination.

NASA approved a plan in 1969 to send a spacecraft to Mercury, utilizing Venus for a gravity assist, which became the Mariner 10 mission. Managed by the Jet Propulsion Laboratory (JPL) in Pasadena, California, this was the final mission in the successful Mariner series of planetary spacecraft, which provided numerous insights into the inner solar system.

Mariner 10 was launched from Cape Canaveral, Florida, on an Atlas Centaur rocket. It had a launch mass of 502 kilograms (1,107 pounds) and was powered by two solar panels. Mariner 10 was equipped with a suite of scientific instruments:

  1. Two Telescopes/Cameras
  2. Infrared Radiometer
  3. Ultraviolet Airglow Spectrometer
  4. Ultraviolet Occultation Spectrometer
  5. Magnetometer
  6. Charged Particle Telescope
  7. Plasma Analyzer
Mariner 10 (artist conception). On March 29, 1974, it performed the first Mercury flyby.
Mariner 10 (artist conception). On March 29, 1974, it performed the first Mercury flyby. It was also the first spacecraft to perform flybys of multiple planets (Venus and Mercury).

Venus flyby

On February 5, 1974, the spacecraft flew within 3,584 miles of Venus, capturing over 4,000 images and significantly advancing our comprehension of the planet hidden by clouds. However, the most significant contribution Venus made to the Mariner 10 mission was its provision of gravitational assistance, redirecting the spacecraft towards Mercury.

Mission planners designed the trajectory so that the encounter with Mercury occurred when the planet was at its furthest point from the Sun in its elliptical orbit, minimizing Mariner 10’s proximity to the Sun and decreasing the risk of overheating. Professor Giuseppe “Bepi” Colombo at the University of Padua in Italy then proposed an innovative trajectory that would allow Mariner 10 to re-encounter Mercury every 176 days, precisely twice the planet’s orbit time around the Sun.

The only limitation of this timing was that Mercury’s rotation period is in a 3:2 resonance with its revolution period around the Sun, indicating that every time Mariner 10 returned to the planet, the same sunlit hemisphere was visible. As a result, only about 40-45% of the planet’s surface was captured in high resolution, allowing for the generation of a detailed map.

The first Mercury flyby in history

On March 16, a mid-course correction was made to refine the trajectory of the spacecraft for optimal scientific measurements during its initial encounter with Mercury. The spacecraft’s first instruments were activated on the following day and it took one week for the first images of the planet to be returned. Although these initial pictures showed about the same amount of detail as those taken from Earth, as the spacecraft approached Mercury, the pictures began to reveal surface features.

On March 29, 1974, Mariner 10 performed the first Mercury flyby, passing only 438 miles (705 km) above Mercury’s surface, and continued to photograph the planet until April 3, resulting in the acquisition of more than 2,000 images and a wealth of data from other scientific instruments.

Mercury appeared heavily cratered and moon-like at first glance, but its surface features showed less contrast than those of the Moon. The planet also displayed other features such as scarps or cliffs that are absent on the Moon, providing hints about its formation. Like the Moon and Mars, Mercury has flat plain-like features or “mares”, which suggest that the three planets have similar ancestral histories involving asteroid bombardment.

Surprisingly, Mariner 10’s magnetometer detected a weak magnetic field on Mercury that is about 1/60th the strength of Earth’s. Radio tracking of the spacecraft’s trajectory also revealed that Mercury is much closer to being a perfect sphere than Earth. The large temperature difference between Mercury’s day and night sides, which exceeds 600 °F (315.5 °C), suggests that its surface is composed of a similar material to the Moon’s, which is a blanket of dust pulverized by meteoric impacts.

Mariner 10 image of Mercury taken during the first flyby.
Mariner 10 image of Mercury taken during the first flyby.

Other Mercury flybys

In total, Mariner 10 made three Mercury flybys, the first on March 29, 1974, the second on September 21, 1974, and the third on March 16, 1975. During each flyby, the spacecraft gathered data on the planet’s surface, atmosphere, and magnetic field. The spacecraft also made measurements of the solar wind and the interplanetary medium.

To properly aim the spacecraft for its second encounter and enable a third one, five mid-course corrections were required. On September 21, 1974, it passed at a distance of 29,875 miles (48,080 km) from Mercury’s sunlit side, which allowed observation of the planet’s south polar region. During the three-day encounter, about 500 new images of the planet were returned, and the greater distance of the flyby enabled scientists to create hemisphere-wide mosaics with amazing detail. The ultraviolet spectrometer on the spacecraft confirmed that Mercury has a very thin atmosphere composed mainly of helium.

The third Mercury flyby, which took place on March 16, 1975, was not without drama in the days leading up to it. Mariner 10, already running low on attitude control fuel, rolled out of communication with Earth. Controllers had to scramble to regain control of the spacecraft and succeeded just in time for the encounter.

This time, the flyby distance was only 203 miles (326.5 km) above the surface, and the main goals were to study the planet’s magnetic field and to take more detailed imagery of sites of interest identified from the first two encounters. About 450 useful narrow strips of photographs were taken, some with a surface resolution down to about 450 feet (137 meters).

Eight days later, Mariner 10 depleted its attitude control fuel, and mission controllers sent a signal to turn the spacecraft off, bringing an end to the flight operations of a highly successful mission that essentially explored two planets for the price of one, and completed four encounters for the price of two. Scientists continued to analyze the data returned by Mariner 10 for many years, and the discoveries made at Venus and especially at Mercury added a great deal to our knowledge of the inner solar system.


M. Özgür Nevres
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